This invention relates to a method of making a self-alignment type metal oxide semiconductor (hereinafter called MOS) device, wherein side-end parts of the oxide film underneath a metal film or a gate conductor film and the surface of source and drain region of a silicon substrate are thermally oxidized in order to improve characteristic of the device.
Conventional methods of making method of the self-alignment type MOS device are elucidated referring to FIGS. 1 and 2.
In FIG. 1, which shows sectional side view of a self-alignment type MOS FET, on a substrate 1 of a single crystal silicon, are sequentially formed thick films of thermally oxidized SiO.sub.2 2 for preventing parasitic MOS effect, a gate insulation film 3 of thin SiO.sub.2 film formed by thermal oxidation, a polycrystalline film 4 to become a gate conductor film when impurity is diffused therein and a CVD-deposited insulation film 8 covering the principal surfaces of the substrate and also the side end parts of the gate insulation film 3. In the substrate are formed impurity-diffused source and drain regions 6 & 7.
The method of making the conventional MOS device of FIG. 1 is as follows: The gate insulation film 3 of a thin SiO.sub.2 film is formed all the way on the substrate 1 of one conductivity and having the thick films 2 at specified parts, and the film 4 to become the gate conductor film of polycrystalline silicon film is formed all the way on the gate insulation film 3. Then the polycrystalline silicon film 4 is etched to have a specified pattern of the gate and connection wirings by utilizing a known photoresist film method. Subsequently, the gate insulation film 3 is etched by utilizing the previously etched polycrystalline silicon film 4 as an etching mask, so as to make openings through which the impurity is diffused to form source and drain regions 6 and 7. The etching is sufficiently carried out in order to completely remove the gate insulation film 3 at the openings. As a consequence of such sufficient etching, the side-end parts of the gate insulation film 3 underneath the polycrystalline silicon film 4 are side-etched thereby making the end parts of the polycrystalline silicon film 4 into a concave shape having hollow parts 5 thereunder. In the conventional MOS device, such hollow parts 5 remain unfilled even after chemical vapor deposition of the SiO.sub.2 film 8 to cover the gate conductor film 4 therewith. Even in case the CVD film 8 is filled in the hollow parts 5, the CVD films 8 in the hollow become porous and of low density which are liable to be contaminated and cause poor electric characteristics, namely of low drain or source reverse breakdown voltages through the gate. When the side-etching underneath the concave shaped gate conductor 4 becomes large, the concave shaped end part 9 of the gate conductor 4 tends to fall down thereby forming cracks thereon, and as a result increasing gate leakage current and deteriorating electric characteristic of the MOS device. As a further consequence of said side-etching, steep steps 81 are formed on the surface of the CVD film 8 near the hollow parts 5, thereby resulting in forming very thin parts in the aluminum wiring film at the steep steps 81. Moreover, the steep steps 81 cause the etchant to retain and excessively etches there which causes a circuit opening of the Al wiring film at the steps.